Ubuntu and edubuntu come with GNOME, kubuntu comes with KDE, and xubuntu comes with Xfce. Below are screenshots of the different distributions.
If you installed one version of ubuntu but would also like a different desktop environment, open your terminal and type the following without quotes and answer "Y" to all the questions it asks.
For ubuntu GNOME "sudo aptitude install ubuntu-desktop"
For edubuntu GNOME "sudo aptitude install edubuntu-desktop"
For kubuntu KDE "sudo aptitude install kubuntu-desktop"
For xubuntu Xfce "sudo aptitude install xubuntu-desktop"
Once installed, you may change sessions before logging in to another desktop.